Metal complex-based electron-transfer mediators in dye-sensitized solar cells

ABSTRACT

This present invention provides a metal-ligand complex and methods for using and preparing the same. In particular, the metal-ligand complex of the present invention is of the formula:
 
L a -M-X b 
 
where L, M, X, a, and b are those define herein. The metal-ligand complexes of the present invention are useful in a variety of applications including as electron-transfer mediators in dye-sensitized solar cells and related photoelectrochromic devices.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority of PCT/US02/34883, filedOct. 30, 2002, now International Publication No. WO 2003/038508, datedMay 8, 2003, which claims the priority benefit of U.S. ProvisionalPatent Application No. 60/335,942, filed Oct. 30, 2001, both of whichare incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

The U.S. Government has paid-up license in this invention and the rightin limited circumstances to require the patent owner to license otherson reasonable terms as provided for by the terms of Grant No.DE-FG03-97ER14808 awarded by the U.S. Department of Energy, and GrantNos. CHE-0139637 and CHE-9714081 awarded by the National ScienceFoundation.

FIELD OF INVENTION

This present invention relates to a metal-ligand complex of the formulaL_(a)-M-X_(b) and methods for using and preparing the same. Inparticular, metal-ligand comlpexes of the present invention are usefulas electron-transfer mediators in dye-sensitized solar cells and relatedphotoelectrochromic devices.

BACKGROUND OF THE INVENTION

The spectral characteristics of wide-bandgap semiconductors have beenenhanced through the use of adsorbed dye molecules. This is referred toas dye sensitization, and cells incorporating these materials asphotoanodes are described as dye-sensitized solar cells (hereafterDSSC's). See, for example, A. Hagfeldt and M. Graetzel, Acc. Chem. Res.,2000, 33, p. 269–277, which is incorporated herein by reference in itsentirety. As a whole, DSSC's are composed of three basic components: thephotoanode, the electron-transfer mediator, and the cathode. A ratherlarge variety of semiconductors and dyes are known to yield effectivephotoanodes. There are, however, a limited number of knownelectron-transfer mediators and compatible cathode materials thatproduce effective solar cells when combined with a known photoanode.See, for example, Lenzmann, et al., J. Phys. Chem. B, 2001, 105, 6352;Magnisson et al., Solar Energy Materials and Solar Cells, 2002, 73,51–58; and Turkovic et al., Solar Energy Materials and Solar Cells,1997, 45, 275–281, all of which are incorporated herein by reference intheir entirety.

Iodide salts of inorganic and organic cations when mixed with iodine(hereinafter I⁻/I₃ ⁻) at varying ratios and concentrations in aproticsolvents are known to be effective electron-transfer mediators inDSSC's. See, for example, Nazeeruddin et al., J. Am. Chem. Soc., 1993,115, p. 6382–90. Comparable bromide salts mixed with bromine (hereafterBr⁻/Br₂) are less effective and less preferred, but are also known. See,for example, Heimer et al., J. Phys. Chem., 1993, 97, p. 11987–94; andVlachopoulos et al., J. Am. Chem. Soc., 1988, 110, p. 1216–20. With boththe I⁻/I₃ ⁻ and the Br⁻/Br₂ mediators, there are a number of drawbacks.Often cell construction is complicated by issues of chemicalcompatibility with the electron-transfer mediator. Both the I⁻/I₃ ⁻ andBr⁻/Br₂ mediator mixtures are highly corrosive. With a few exceptions,notably titanium or platinum, the use of metals must be avoided forsuccessful long-term operation of the cells. The volatility of bothiodine and bromine further complicates the sealing of cells, and leakageis often the cause of device failure.

There has also been a report of a cobalt complex-based mediator thatrivaled the I⁻/I₃ ⁻ mediator in terms of the kinetics to regenerate thedye. Nusbaumer et al., J. Phys. Chem. B, 2001, 105, 10461. However, theligands used to form this complex are not readily available, and arebelieved to require a multi-step synthetic procedure, thereby addingtime and cost in producing these electron-transfer mediators. Many otherprevious efforts in cobalt complexes as mediators in DSSCs have alsobeen not too successful. See, for example, Bonhote et al., Presented atthe 10th International Conference on Photochemical Conversion andStorage of Solar Energy (IPS-10), Interlaken, Switzerland, 1994,Abstract C2; and Wen et al., Sol. Energy Mater. Sol. Cells, 2000, 61,339.

Platinum and titanium are known to be compatible cathode materials,being resistant to the corrosive nature of the above electron-transfermediators. Platinum is generally most preferred because its surface isknown to be catalytic for the reduction of iodine to iodide. Gold,silver, nickel, iron, chromium, aluminum, and copper (along with mostother metals and alloys thereof) cannot be used with I⁻/I₃ ⁻. Thislimitation has been one of the major obstacles in the commercializationof DSSC's.

Passivation with electrically insulating materials of one or more of theactive surfaces in the photoanode is known to allow the use of otherelectron-transfer mediators such as ferrocene/ferrocenium. See, forexample, Gregg et al., J. Phys. Chem. B, 2001, 105, p. 1422–1429.However, the performance of these cells does not approach that of cellsusing I⁻/I₃ ⁻. Additionally, the passivation methods are known to bedifficult to control and reproduce.

The operation of the dye-sensitized photoanode in certainphotoelectrochromic devices is subject to most of the sameconsiderations as DSSC's. See, for example, Pichot et al., J.Electrochem. Soc., 1999, 146, p. 4324–4326, which is incorporated hereinby reference in its entirety. Consequently, similar advantages will beaccrued by replacing the I⁻/I₃ ⁻ electron-transfer mediator system withthe present invention in photoelectrochromic devices.

Therefore, there is a need for other simple and efficientelectron-transfer mediators.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a metal-ligand complex ofthe formula:L_(a)-M-X_(b)  Iwherein a is an integer from 1 to 6; b is an integer from 0 to 5,provided the sum of a and b equal the appropriate total number ofligands present on the metal M; M is a transition metal; each X isindependently a co-ligand; and each L is independently a polypyridineligand.

The present invention also provides methods for using and makingcompounds of Formula I above. In one particular embodiment, the presentinvention provides an electron-transfer mediator comprising themetal-ligand complex of Formula I above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows some of the representative electron-transfer mediators ofthe present invention which comprises a series of terpyridine,bipyridine, and phenanthroline complexes of cobalt(II). The abbreviatednames, as used in the text, refer to the entire complex, not theligands.

FIG. 2 shows cyclic voltammograms of three different cobalt complexes(in rows) on three different electrodes (in columns). The vertical axisis current density to compensate for modest differences in electrodeareas, and the scale is indicated in the upper left hand corner. Theconcentration of the complexes was 10⁻³ M, and the scan rate was 200 mVs⁻¹.

FIG. 3 is a photoaction spectra of N3 bound to nanocrystalline TiO₂films in the presence of different electron mediators in MPN solutions:0.25 M LiI/25 mM I₂ (

), 0.25 M ttb-terpy²⁺/25 mM NOBF₄

, 0.25 M dtb-bpy²⁺/25 mM NOBF₄ (

), 0.25 M phen²⁺/25 mM NOBF₄ (

), 0.25 M te-terpy²⁺/25 mM NOBF₄ (

), saturated (<0.15 M) 4,4′-dmb²⁺/15 mM NOBF₄ (

). 0.25 M LiClO₄ was added to all solutions containing a cobaltmediator.

FIG. 4 is a plot of V_(oc), E_(1/2), and J_(sc) as a function of thenumber of carbon atoms in the alkyl or aryl substituents at the 4 and 4′positions of 2,2′-bipyridine ligands. V_(oc) and J_(sc) were measured inDSSCs with a gold cathode and containing 125 mM Co(II)L₃ and 13 mM NOBF₄in MPN.

FIG. 5 shows current-voltage response of DSSCs with a gold cathode andcontaining: 0.25 M dtb-bpy and 25 mM NOBF₄ in gBL (-..-), with added 0.2M 4-t-butylpyridine (-.-), with added 0.2 M 4-t-butylpyridine and 0.2 Mlithium triflate (- -), and with added 0.2 M 4-t-butylpyridine and 0.5 Mlithium triflate (-).

FIG. 6 shows current-voltage response of DSSCs assembled from N3-dyedphotoanodes of acetic acid prepared TiO₂. These photoanodes were treatedwith a solution 0.5 M 4-t-butylpyridine in ACN just prior to use. Thesolid line represents a mediator of 0.5 M LiI and 50 mM I₂ in MPN usinga platinum cathode. The dashed line represents a mediator of saturated(<0.5 M) dtb-bpy, 50 mM NOBF₄, and 0.5 M lithium triflate in gBL using agold cathode.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The term “alkyl” refers to a linear or branched saturated monovalenthydrocarbon moiety, preferably having from one to about 12 carbon atoms.Exemplary alkyl groups include methyl, ethyl, n-propyl, 2-propyl,tert-butyl, pentyl, 3-pentyl, hexyl, heptyl, octyl, nonyl, and the like.

The term “alkylene” refers to a linear or branched saturated divalenthydrocarbon moiety, preferably having from one to about 12 carbon atoms.Exemplary alkylene groups include methylene, ethylene, n-propylene,2-propylene, tert-butylene, pentylene, 3-pentylene, hexylene, heptylene,octylene, nonylene, and the like.

The term “aryl” refers to a monovalent aromatic hydrocarbon ring moiety,such as mono-, bi- or tri-cyclic aromatic carbocyclic ring moieties.Exemplary aryls include, but are not limited to, phenyl and naphthyl.Aryl groups can optionally be substituted with one or more substituentssuch as alkyl, halo, hydroxyl, or alkoxy.

The term “aralkyl” refers to a moiety of the formula —R′R″, where R′ isalkylene and R″ is aryl as defined herein.

The term “carboxy” refers to a moiety of the formula —C(═O)—OR^(a),where R^(a) is hydrogen, alkyl, cycloalkyl, haloalkyl, aryl, or aralkyl.Preferably R^(a) is alkyl. And the term “alkyl carboxy” refers to acarboxy moiety as defined herein where R^(a) is alkyl.

The term “carboxylate” refers to a moiety of the formula —OC(═O)R^(z),where R^(z) is hydrogen, alkyl, cycloalkyl, aryl, or aralkyl.

The term “cycloalkyl” refers to a mono- or bicyclic saturated monovalenthydrocarbon moiety, preferably having from three to about 12 carbonatoms. Exemplary cycloalkyl groups include cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclododecyl,bicyclo[4.4.0]decyl, and the like.

The term “amide” refers to a moiety of the formula —C(═O)—NR^(b)R^(c),where each of R^(b) and R^(c) is independently, hydrogen, alkyl,cycloalkyl, haloalkyl, aryl, or aralkyl. Preferably, R^(b) and R^(c) areindependently hydrogen or alkyl.

The term “haloalkyl” refers to an alkyl group in which one or morehydrogen atom has been replaced by halide. Exemplary haloalkyl groupsinclude —CH₂Cl, —CF₃, —CH₂CF₃, —CH₂CCl₃, and the like, and furtherincludes those alkyl groups such as perfluoroalkyl in which all alkylhydrogen atoms are replaced by fluorine atoms.

The term “polypyridine” refers to a moiety comprising two or morepyridine ring system which are covalently linked to one another. Each ofthe covalently linked pyridine ring system can optionally be substitutedwith alkyl, halo, haloalkyl, carboxy, amide, or aryl. In addition, thecovalently linked pyridine ring systems can together form a larger ringsystem. Preferred polypyridines include bipyridine, terpyridine,phenanthroline, and the derivatives thereof.

The terms “bidentate polypyridine”, “tridentate polypyridine”, and“tetradentate polypyridine” refer to a polypyridine moiety having two,three, and four pyridine nitrogen atoms, respectively, which arecoordinated to the metal.

As used herein, the terms “those defined above” and “those definedherein” when referring to a variable incorporates by reference the broaddefinition of the variable as well as preferred, more preferred and mostpreferred definitions, if any.

Compounds of the Present Invention

The present invention provides a metal-ligand complex comprising amoiety of the formula:L_(a)-M-X_(b)  (Formula I)where M is a transition metal, preferably cobalt, and more preferablycobalt having oxidation state of 2 or 3; a is an integer from 1 to 6,preferably 2 or 3; b is an integer from 0 to 5, preferably 0; each X isindependently a co-ligand; and each L is independently a polypyridineligand, provided that the sum of a and b equal the appropriate totalnumber of ligands present on the metal M. For example, when M contains atotal of six ligand binding sites and L is a bidentate ligand, then thesum of 2a+b is 6, and when M contains four ligand binding sites and L isa tridentate ligand, then 3a+b is 4, i.e., a and b are 1.

Preferably, each L is independently a bidentate, a tridentate or atetradentate polypyridine ligand. More preferably, each L isindependently a bidentate or a tridentate polypyridine ligand. Stillmore preferably, each L is independently terpyridine, bipyridine, orphenanthroline, each of which is optionally substituted. In oneembodiment, L comprises at least one substituent which has a stericvolume larger than a methyl group.

Preferably, the tridentate polypyridine ligand is optionally substitutedterpyridine. More preferably, the tridentate polypyridine is of theformula:

where each R¹ is independently hydrogen, alkyl, cycloalkyl, aryl,haloalkyl, heteroaryl, carboxy, or amide. Preferably, each R¹ isindependently hydrogen, alkyl, or haloalkyl. More preferably, each R¹ isindependently hydrogen or alkyl. Still more preferably, at least one R¹is alkyl. And more preferably, each R¹ is alkyl. Still yet morepreferably, each R¹ is independently selected from the group consistingof ethyl and tert-butyl. In one specific embodiment, R¹ is ethyl. Inanother specific embodiment, R¹ is tert-butyl.

Preferably, the bidentate polypyridine ligand is optionally substitutedbipyridine or optionally substituted phenanthroline. More preferably,the bidentate polypyrdine is of the formula:

where each of R², R³, R⁴, and R⁵ is independently hydrogen, alkyl, aryl,carboxy, amide, cycloalkyl, haloalkyl, or heteroaryl.

Preferably, each R² is independently selected from the group consistingof hydrogen, alkyl, aryl, carboxy, and amide. More preferably, each R²is independently selected from the group consisting of hydrogen, alkyl,N,N-dialkyl amide, alkyl carboxy, and phenyl. Still more preferably,each R² is independently selected from the group consisting of hydrogen,methyl, tert-butyl, 3-pentyl, nonyl, N,N-dibutyl amide, tert-butylcarboxy, and phenyl.

Preferably, each R³ is independently selected from the group consistingof hydrogen, alkyl, aryl, carboxy, and amide. More preferably, each R³is independently hydrogen or alkyl. Still more preferably, each R³ isindependently hydrogen or methyl.

Preferably, each R⁴ is independently selected from the group consistingof hydrogen, alkyl, aryl, carboxy, and amide. More preferably, each R⁴is independently hydrogen or aryl. Yet more preferably, each R⁴ isindependently hydrogen or optionally substituted phenyl. Still morepreferably, each R⁴ is independently hydrogen or phenyl.

Preferably, each R⁵ is independently selected from the group consistingof hydrogen, alkyl or haloalkyl. More preferably, each R⁵ isindependently hydrogen or alkyl. Still more preferably, R⁵ is hydrogen.

Still more preferably, at least one of the substituents of thepolypyridine ligand of Formulas II, III, and IV is a substituent otherthan hydrogen or methyl. And more preferably, at least one of thesubstituents of the polypyridine ligand of Formulas II, III, and IV is asubstituent having a steric volume greater than a methyl group.

It is to be understood that the scope of this invention encompasses notonly the various polypyridine ligand isomers which may exist but alsothe various mixture of polypyridine ligand isomers which may be formeddepending on the substituents that are present on the polypyridineligand.

In particular, if the metal-ligand complex of the present inventioncontains one or more chiral centers, the metal-ligand complex can besynthesized enantioselectively or a mixture of enantiomers and/ordiastereomers can be used as is or prepared and separated. Theresolution of the compounds of the present invention, their startingmaterials and/or the intermediates can be carried out by any of themethods known to one skilled in the art. See for example, OpticalResolution Procedures for Chemical Compounds: Optical ResolutionInformation Center, Manhattan College, Riverdale, N.Y., and inEnantiomers, Racemates and Resolutions, Jean Jacques, Andre Collet andSamuel H. Wilen; John Wiley & Sons, Inc., New York, 1981, which areincorporated herein in their entirety. Basically, the resolution of thecompounds is based on the differences in the physical properties ofdiastereomers by attachment, either chemically or enzymatically, of anenantiomerically pure moiety results in forms that are separable byfractional crystallization, distillation or chromatography.

When one or more co-ligand is present in the metal-ligand complex, eachco-ligand, X, is independently selected from a conventional transitionmetal ligands known to one skilled in the art. Exemplary co-ligandsinclude halide, alkyl, carboxylate, nitro, nitroso, a phosphinederivative (such as triarylphosphine, trialkylphosphine, etc.), and thelike.

Still further, combinations of the preferred groups described hereinform other preferred embodiments. For example, in one particularlypreferred embodiment M is cobalt, L is terpyridine of formula II and R¹is tert-butyl. In this manner, a variety of preferred compounds areembodied within the present invention.

It should be appreciated that when the metal, M, is not a neutralspecies, the metal-ligand complex constitutes only a partialrepresentation of a chemical compound. Indeed, in an isolable compound,such a moiety must be paired with a counterion (e.g., anion or cation)that is necessary to maintain electroneutrality. Thus, compounds of thepresent invention are, more accurately, represented by the formula:[(L)_(a)-M-(X)_(b)]_(m)Y_(n)  (Formula IA)where L, M, X, a and b are those defined above, Y is a counterion andthe variables m and n are oxidation state of the counterion and themetal-ligand complex, respectively, with a proviso that when themetal-ligand complex is not charged no counterion will be present andn=0 and m is 1. While Y can sometimes affect the solubility or othernon-electrochemical property of the metal-ligand complex, the exactnature of Y is not critical.

Preferred compatible anions, i.e., Y, are: ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻,p-toluenesulfonate⁻, NO₃ ⁻ and trifluoromethanesulfonate. With ClO₄ ⁻anion being a preferred counter anion. Preferred compatible cations,i.e., Y, are: Li⁺, Na⁺, K⁺, NH₄ ⁺ and NR₄ ⁺ where R is a straight chainalkane containing 1 through 8 carbons. However, the present invention isnot limited to these counterions.

If desired, a counter-ion associated with a metal-ligand complex cation(or anion) can be readily exchanged with another counter-ion by any ofthe methods known to one skilled in the art, including ion exchangechromatography and other ion exchange methods.

Synthesis

The metal-ligand complex of the present invention can be synthesizedfrom readily available starting materials. Typically, a polypyridinecompound is dissolved in an organic solvent and an appropriate amount ofmetal salt is added. The mixture is then stirred at a temperature andfor a period sufficient to affect exchange of ligands to produce thedesired metal-ligand complex. The reaction temperature can range fromroom temperature to the boiling point of the solvent used.

Typically, the ligand exchange reaction is carried out by refluxing themixture. Suitable reaction solvents include alcohols (e.g., methanol,ethanol, isopropanol, etc.), and other solvents well known to oneskilled in the art.

The reaction time can vary depending on a variety of factors, includingthe polypyridine compound and the metal salt used. Other factors includereaction solvent, reaction temperature, and concentrations of eachreaction components. The reaction time generally ranges from few minutesto few hours. Typically, the reaction time ranges from 1 to 5 hours.

The reaction conditions are not limited to those described above andexamples given herein. The reaction conditions can vary depending on theparticular reaction solvent, polypyridine, and metal salt used in orderto affect the desired ligand exchange reaction.

Various polypyridine ligand compounds are commercially available or canbe readily obtained from commercially available starting materials.Thus, various substituents on the polypyridine ligand of the presentinvention can be present in the starting compounds, added to any one ofthe intermediates or added after formation of the final products byknown methods of substitution or conversion reactions. If thesubstituents themselves are reactive, then the substituents canthemselves be protected according to the techniques known in the art. Avariety of protecting groups are known in the art, and can be employed.Examples of many of the possible groups can be found in ProtectiveGroups in Organic Synthesis, 3rd edition, T. W. Greene and P. G. M.Wuts, John Wiley & Sons, New York, 1999, which is incorporated herein byreference in its entirety. For example, nitro groups can be added to anaromatic ring system by nitration and the nitro group can be convertedto other groups, such as amino by reduction, and halogen bydiazotization of the amino group and replacement of the diazo group withhalogen. Acyl groups can be added by Friedel-Crafts acylation. The acylgroups can then be transformed to the corresponding alkyl groups byvarious methods, including the Wolff-Kishner reduction and Clemmensonreduction. Amino groups can be alkylated to form mono- and di-alkylaminogroups; and mercapto and hydroxy groups can be alkylated to formcorresponding ethers. Primary alcohols can be oxidized by oxidizingagents known in the art to form carboxylic acids or aldehydes, andsecondary alcohols can be oxidized to form ketones. Thus, substitutionor alteration reactions can be employed to provide a variety ofsubstituents throughout the molecule of the starting material,intermediates, or the final product, including isolated products.

Since the compounds of the present invention can have certainsubstituents which are necessarily present, the introduction of eachsubstituent is, of course, dependent on the specific substituentsinvolved and the chemistry necessary for their formation. Thus,consideration of how one substituent would be affected by a chemicalreaction when forming a second substituent would involve techniquesfamiliar to one of ordinary skill in the art. This would further bedependent on the nature of the polypyridine involved.

Utility

Certain photoelectrochemical cells based on dye-sensitizednanocrystalline TiO₂ photoanodes can have total energy conversionefficiency in excess of 10% when irradiated with sunlight. See, forexample, Nazeeruddin et al., J. Am. Chem. Soc., 2001, 123, 1613. Suchefficiencies meet or exceed those of solid-state cells based onamorphous silicon but fall far short of the efficiency of single crystaland poly-crystalline silicon cells. Green, MRS Bull., 1993, 18, 26;Watanabe, MRS Bull., 1993, 18, 29; and Hamakawa et al., MRS Bull., 1993,18, 38. That fact notwithstanding, the potential for fabricating largesurface area cells out of relatively inexpensive materials—compared tosingle crystalline silicon cells, for example—is driving interest indye-sensitized solar cells (DSSCs).

While the demonstrated energy conversion efficiencies of DSSCs havebecome competitive with some existing commercial technologies, there area number of issues that remain to be addressed before this type of cellcan become truly commercially viable. Currently, the best “dyes” forsensitizing the TiO₂ photoanode are ruthenium-based coordinationcomplexes. With such dyes there are potential stability issues.Furthermore, ruthenium is relatively rare. To date, only cells based onliquid-state electrolytes have produced the high efficiencies requiredfor competitiveness with existing technologies. Cao et al., J. Phys.Chem., 1995, 99, 17071; Papageorgiou et al., J. Electrochem. Soc., 1996,143, 3099; Murakoshi et al., Chem. Lett., 1997, 471; Murakoshi et al.,Sol. Energy Mater. Sol. Cells, 1998, 55, 113; and Savenije et al., Chem.Phys. Lett., 1998, 287, 148. Unfortunately, an extremely limited set ofelectron-transfer mediators work in these cells. The overall best systemto date is the I⁻/I₃ ⁻ couple, which has a list of undesirable chemicalproperties. Gregg et al., J. Phys. Chem. B, 2001, 105, 1422.

Considerable effort has been focused on finding new dyes. Hagfeldt etal., Acc. Chem. Res., 2000, 33, 269. In contrast, efforts to findelectron-transfer mediators other than I⁻/I₃ ⁻ have been relativelymodest. Gregg et al., J. Phys. Chem. B, 2001, 105, 1422; and Oskam etal., J. Phys. Chem. B, 2001, 105, 6867. The I⁻/I₃ ⁻ couple functionswell in these cells because of a fortunate confluence of the rightkinetics for at least four different heterogeneous electron-transferreactions: (1) The photo-excited dye must inject an e⁻ faster than itreacts with the mediator. (2) The oxidized dye must be reduced by themediator more rapidly than it recombines with the photoinjectedelectron. (3) The oxidized mediator must, itself, react slowly withelectrons in both the TiO₂ and the fluorine-doped tin oxide (SnO₂:F)contact. (4) Finally, the reduction of the oxidized mediator at thecathode must be rapid.

Present inventors have discovered that metal-ligand complexes of thepresent invention—some of which are formed from structurally simpleligands—function as efficient electron-transfer mediators in DSSCs. FIG.1 shows a representative metal-ligand complexes of the presentinvention.

When metal-ligand complexes of the present invention are used aselectron-transfer mediators, efficiencies of such electron-transfer isat least about 50%, preferably at least about 70% and more preferablygreater than 80%, of that given by the comparable cell mediated by theI⁻/I₃ ⁻ couple.

Thus, metal-ligand complexes of the present invention are useful in avariety of application, including as electron-transfer mediators,especially in dye-sensitized solar cells (e.g., batteries) and relatedphotoelectrochromic devices. As stated above, conventional solar cellstypically utilize I⁻/I₃ ⁻ system as an electron-transfer mediator.Unfortunately, I⁻/I₃ ⁻ system is corrosive and I₂ which is a componentof the I⁻/I₃ ⁻ system is volatile.

In contrast, metal-ligand complexes of the present invention aresignificantly less corrosive than the I⁻/I₃ ⁻ system and significantlyless volatile than I₂. Thus, use of non-volatile and non-corrosivemetal-ligand complexes of the present invention enable the facilefabrication of solar cells, including solar cells containing cathodes ofmaterials other than platinum or titanium, and allows for long-termstability in sealed cells.

The electron-transfer mediators of the present invention areredox-active metal complexes. Such electron-transfer mediators can beused in conjunction with or in place of other conventionalelectron-transfer mediators, including I⁻/I₃ ⁻ system.

Typically, when simple outer-sphere type redox agents are used aselectron-transfer mediators in DSSC's (in place of I⁻/I₃ ⁻), the rate ofheterogeneous electron transfer between the oxidized form of themediator molecule and either the semiconductor particle surface or thesurface of the electrical contact to the semiconductor-particle film, orboth (i.e., “undesired reactions”), is significantly faster than therates of mass transfer of the oxidized mediator to the cathode or theheterogeneous reduction of the oxidized mediator at the cathode (i.e.,“desired reactions”). As a result of these undesired reactions, thephotocurrents generated by cells employing such mediators are small andtheir light-to-electrical-energy conversion efficiencies are poor whencompared to identical cells employing I⁻/I₃ ⁻ as the electron-transfermediator.

In contrast, without being bound to any theory, it is believed that themetal-ligand complexes of the present invention have built into theirstructure a means or multiple means to significantly reduce the rates ofthese undesired surface reactions. This reduction in undesired reactionrate is typically accomplished without any special chemical or physicaltreatment of the semiconductor or electrical contact surfaces that mightalso serve to slow the undesired reverse electron transfer reactions. Itis believed that the desired redox reactions of these complexes with theoxidized dye on the semiconductor surface and the cathode remain fastenough for the cell to function with reasonable efficiencies. Thus,highly efficient solar cells are obtained by using a metal-ligandcomplex of the present invention.

Electron-transfer mediator systems comprising a metal-ligand complex ofthe present invention function efficiently under a wider range ofsolvent conditions. Furthermore, since metal-ligand complexes of thepresent invention are significantly less corrosive than I⁻/I₃ ⁻ system,they are compatible with a wider range of potential electrode materialsthan I⁻/I₃ ⁻ system. In addition, a relatively non-corrosive propertyallows metal-ligand complex of the present invention to be used as anelectron-transfer mediator system that is compatible with a wide varietyof potential structural materials, such as metals, plastics and otherpolymers known to one skilled in the art. Moreover, due to itsrelatively non-volatile nature, metal-ligand complexes of the presentinvention can be used as electron-transfer mediator systems in gelelectrolytes, in the solid state, or in polymeric form.

In application as an electron-transfer mediator in DSSC's, the preferreduse is a combination of the two oxidation-state forms of the complexwith otherwise the same structure. The most preferred combination is 10%n=3 and 90% n=2, where the percentages refer to the relative amounts ofeach oxidation-state form of the metal-ligand complex.

EXAMPLES

The following examples are offered to illustrate, but not to limit theclaimed invention.

Materials. Acetonitrile (Fisher Optima Grade), anhydrous ethanol(Pharmco), and all other solvents (Fisher ACS Grade) were used asreceived. Cobalt(II) perchlorate hexahydrate, 10% palladium on activatedcarbon, γ-butyrolactone, thionyl chloride, t-butyl alcohol,dibutylamine, 4-t-butylpyridine, 2,2′-dipyridyl,4,4′-diphenyl-2,2′-dipyridyl, 4,4′-di-t-butyl-2,2′-dipyridyl,4,4′-dinonyl-2,2′-dipyridyl, 4,4′,4″-tri-t-butyl-2,2′:6′,2″-terpyridine,1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, silver nitrate,lithium perchlorate, lithium triflate, and nitrosonium tetrafluoroboratewere purchased from Aldrich at ≧97% purity and used as received.3-methoxypropionitrile (Aldrich 98%) and 4-(3-pentyl)pyridine (TCI 95+%)were distilled under dynamic vacuum prior to use.4,4′-Dimethyl-2,2′-dipyridyl was purchased from Reilly Industries(Indianapolis, Ind.). 5,5′-Dimethyl-2,2′-dipyridyl, and4,4′,5,5′-tetramethyl-2,2′-dipyridyl were prepared using the procedureas described by Sasse et al., in J. Am. Chem. Soc. 1961, 83, 1347. Allmethyl-substituted bipyridines were recrystallized from ethyl acetate.2,2′-Bipyridine-4,4′-dicarboxylic acid was prepared using the procedureas described by Nazeeruddin et al., in Inorg. Synth. 1997, 32, 181.4,4′,4″-Triethyl-2,2′:6′,2″-terpyridine was prepared using the procedureas described by Nazeeruddin et al., in J. Am. Chem. Soc. 2001, 123,1613. Cis-di(isothiocyanato)-bis(2,2′-bipyridine-4,4′-dicarboxylicacid)ruthenium(II) (i.e., “N3”) was prepared using the procedure asdescribed by Nazeeruddin et al., in J. Am. Chem. Soc. 1993, 115, 6382.

Example 1

This example illustrates a method for synthesizing4,4′-di-(3-pentyl)-2,2′-dipyridyl.

Freshly distilled 4-(3-pentyl)pyridine (18 mL) was refluxed with 2 g of10% Pd on activated carbon under nitrogen for 5 days. After cooling themixture to room temperature, the solids were filtered and rinsed withdichloromethane (CH₂Cl₂). The solvent in filtrate was then removed byrotary evaporation. Most of the unreacted 4-(3-pentyl)pyridine wasremoved by vacuum distillation. The remaining viscous oil was subjectedto flash column chromatography using silica gel. The eluent was agradient of acetone in CH₂Cl₂ that was saturated with concentratedammonium hydroxide solution. Combining and reducing the volume of theproduct containing fractions resulted in 2.7 g (16% yield) of4,4′-di-(3-pentyl)-2,2′-dipyridyl in the form of a nearly colorlessviscous oil that solidified into a waxy crystalline solid on standing atroom temperature.

¹H NMR (300 MHz, CDCl₃) δ ppm: 0.81 (12H, t, 4×CH₃), 1.73 (8H, m,4×CH₂), 2.50 (2H, m, 2×CH), 7.16 (2H, s, 2×H5), 8.34 (2H, s, 2×H3), 8.61(2H, d, 2×H6).

Example 2

This example illustrates a method for synthesizing2,2′-bipyridine-4,4′-dicarboxylic acid chloride.

Approximately 10 g of 2,2′-bipyridine-4,4′-dicarboxylic acid was placedin a 500 ml round-bottom flask fitted with a condenser. Thionyl chloride(ca. 200 ml) was added and the flask flushed with N₂. The solution wasrefluxed under a static N₂ atmosphere with stirring for three to fourdays. The solution was allowed to cool and the solids settle.Approximately 50 ml of the clear yellowish solution was decanted into aclean 250 ml flask being careful not to transfer any of the un-reactedsolid. The thionyl chloride was removed by rotary evaporation leaving aslightly yellow-green solid on the sides of the flask. This product wasused immediately without characterization or further purification.

Example 3

This example illustrates a method for synthesizing2,2′-bipyridine-4,4′-di-t-butoxyester.

From a freshly opened bottle that had previously been warmed to melt thecontents, ca. 20 ml of t-butyl alcohol was transferred to a clean dryErlenmeyer flask. A piece of sodium (ca. 1 g) was washed several timeswith t-butyl alcohol and added to the Erlenmeyer flask. The flask waswarmed with stirring under N₂ until the sodium had totally dissolved(ca. 1.5 hrs). This solution of sodium t-butoxide was then added to theflask containing the 2,2′-bipyridine-4,4′-dicarboxylic acid chloride.The flask immediately became hot to the touch. The resulting slurry wasstirred for 30 min. and allowed to cool to room temperature. Thesolution was then filtered and the solid was washed with severalportions of CH₂Cl₂.

The solution fractions were combined and the solvent removed by rotaryevaporation leaving a yellowish solid on the sides of the flask. Thissolid consisted of the desired product, the monoacid-monoesterbipyridine and a small amount of dicarboxylic acid bipyridine. Thedesired product was extracted from the solid mixture by adding several10 ml portions of toluene to the flask and heating with swirling with aheat gun until the start of reflux. The toluene was then allowed to coolto room temperature before decanting from the solid residue. Thisprocesses was repeated until the toluene no longer tested significantlypositive for dissolved bipyridine (by adding several drops to a solutionof Fe(ClO₄)₂.X H₂O in acetone which turns red-purple if the product ispresent). The toluene fractions were combined, the volume reduced to afew milliliters, and the solution was placed in a freezer. White, waxycrystals formed and were filtered from the cold solution. The productthus obtained was pure by TLC (˜1 g).

¹H NMR (300 MHz, CDCl₃) δ ppm: 1.65 (18H, s, 6×CH₃), 7.85 (2H, d, 2×H5),8.84 (2H, m, 2×H3, 2×H6).

Example 4

This example illustrates a method for synthesizing2,2′-bipyridine-4,4′-bis-(di-n-butylamide).

Approximately 20 ml of di-n-butylamine was added to a flask containingthe 2,2′-bipyridine-4,4′-dicarboxylic acid chloride and the flaskswirled for several minutes. After the reaction mixture cooled,approximately 100 ml of chloroform was added to the flask. This solutionwas extracted several times with aqueous NaOH. The organic layer wascollected, dried with anhydrous sodium carbonate and reduced to a fewmilliliters. The crude product was chromatographed on silica gel using agradient of acetone in CH₂Cl₂. The product obtained by removing thechromatography solvent was a white residue that had to be scraped fromthe sides of the flask. Estimated yield was ˜1 g.

¹H NMR (300 MHz, CDCl₃) δ ppm: 0.81 (6H, t, 2×CH₃), 1.02 (6H, t, 2×CH₃),1.15 (4H, two offset quintets, 2×CH₂), 1.43 (4H, two offset quintets,2×CH₂), 1.52 (4H, p, 2×CH₂), 1.68 (4H, p, 2×CH₂), 3.20 (4H, t, 2×CH₂),3.51 (4H, t, 2×CH₂), 7.30 (2H, m, 2×H5), 8.41 (2H, s, 2×H3), 8.74 (2H,m, 2×H6).

Example 5

This example illustrates a method for synthesizing [Co^(II)(L)₃]{ClO₄}₂and [Co^(II)(L′)₂]{ClO₄}₂ complexes.

All of the complexes depicted in FIG. 1 were synthesized using the sameprocedure. Briefly, 3 equivalents of a bidentate ligand or 2 equivalentsof a tridentate ligand were dissolved with magnetic stirring inrefluxing methanol. The volume of methanol was adjusted according to thesolubility of the ligand and the scale of the reaction such that all ofthe ligand material was dissolved. To this mixture was then added 1equivalent of cobalt(II) perchlorate hexahydrate and the mixture wasallowed to stir at reflux for 2 hours. After cooling the mixture to roomtemperature, the total volume was reduced by ca. 80% using rotaryevaporation. Addition of ethyl ether caused the precipitation of theproduct (as a solid that varied from light brown to light yellow), whichwas filtered and dried under vacuum. The resulting complexes were usedwithout any further purification.

Example 6

This example illustrates a method for preparing a dye solution.

Saturated solutions of N3 were prepared by adding ca. 4 mg of dye to 10ml of dry ethanol. This mixture was sonicated for ca. 10 minutes andfiltered to remove undissolved dye.

Example 7

This example illustrates a method for preparing an electrode.

TiO₂ colloidal solutions were prepared either according to “Method A”reported by Nazeeruddin et al. (J. Am. Chem. Soc., 1993, 115, 6382) oraccording to the method reported by Zaban et al. (J. Phys. Chem. B,1997, 101, 55) and will be referred to as the nitric acid or acetic acidpreparation, respectively. Films of the colloid were coated onto SnO₂:Fcoated glass electrodes (Pilkington TEC 15) using the “1 Scotch” methodas described by Zaban et al. (J. Phys. Chem. B, 1997, 101, 55). Aftercoating, the films were air-dried and then sintered in air at 450° C.for 1 hour. The still hot electrodes (ca. 80° C.) were then immersed inthe dye solution and allowed to sit in the dark at least overnight.Photoanodes were kept in the dark and in the dye solution until needed.Just prior to use, they were removed from the dye solution, rinsedthoroughly with dry ethanol, and dried under a stream of nitrogen. Insome cases, the photoanodes were further treated by immersing into a 0.5M solution of 4-t-butylpyridine in acetonitrile (ACN) for 10–30 minutesfollowed by rinsing in ACN just prior to use.

Platinum-on-glass electrodes were made by a sputtering process.Gold-on-glass electrodes were made by thermal vapor deposition of 25 nmchromium followed by 150 nm gold on glass. Carbon-coated electrodes weremade by spraying 3–5 coats of Aerodag G (Acheson) on SnO₂:F electrodes.The carbon coating produced in this way was very fragile, and eachelectrode was used in a cell once, as cell disassembly usually createdlarge scratches in the carbon film.

Example 8

This example illustrates a method for preparing electron-transfermediators.

Cobalt-based electron-transfer mediators were created by the addition ofthe desired Co(II) complex at various concentrations in eithermethoxypropionitrile (MPN) or γ-butyrolactone (gBL). In all cases, theappropriate amount of nitrosonium tetrafluoroborate (NOBF₄) was added tooxidize 10% of the added Co(II) complex. In some cases, 0.2 M4-t-butylpyridine was added to the mediator solutions. Lithium triflateor LiClO₄ was also added at various concentrations to some mediatorsolutions. For the purposes of comparison, a standard iodide-basedmediator solution was prepared that consisted of LiI and I₂ (10:1) inMPN.

Example 9

This example illustrates a method for measuring performance ofelectron-transfer mediators.

UV-vis spectra were obtained using a HP 8452A diode arrayspectrophotometer. A reduced volume, 1 cm path length, quartz cell wasused for measurement of all solutions. Cyclic voltammetric data wasobtained using a standard three-electrode cell with an EG&G PAR Model173 Potentiostat/Galvanostat controlled by a Model 175 UniversalProgrammer. The data was recorded on a Yokogawa 3023 X-Y recorder. Thereference electrode was Ag/Ag⁺ (0.47 V vs. SHE) composed of 0.1 M silvernitrate in dimethylsulfoxide. The auxiliary electrode was a 0.5 cm²platinum flag and the working electrode was a glassy carbon (7.1×10⁻²cm²), gold (7.1×10⁻² cm²), or platinum (2.8×10⁻² cm²) disk electrode(BAS). Prior to use, each working electrode was polished on a felt padwith a water slurry of 0.3 μm alumina polishing powder, followed byrinsing and sonication in ACN. This polishing procedure was repeatedbefore each electrochemical experiment. The supporting electrolyte was0.1 M lithium perchlorate in ACN.

Photoaction spectra were obtained from DSSCs in a two-electrode sandwichcell arrangement. Typically 10 μl of electrolyte was sandwiched betweena TiO₂ photoanode and a counter electrode. When solutions of thedifferent cobalt mediators (0.25 M Co(II)/0.025 M NOBF₄) in MPN wereused, the counter electrode was made of gold-sputtered on SnO₂:F-coatedglass. A platinum-sputtered SnO₂:F-coated glass electrode was employedas a counter electrode when the redox mediator was 0.25 M LiI/0.025 MI₂. The cell was illuminated with a 150 W Xe lamp coupled to an AppliedPhotophysics high irradiance monochromator. The irradiated area was 0.5cm². Light excitation was through the SnO₂:F-coated glass substrate ofthe photoanode. Photocurrents were measured under short circuitconditions with a Contron model DMM 4021 digital electrometer. Incidentirradiance was measured with a calibrated silicon photodiode from UDTTechnologies.

To test the performance of each electron-transfer mediator solution,cells were assembled by clamping together a photoanode and cathode in acell holder having a light aperture area of 0.4 cm². Theelectron-transfer mediator was introduced by the addition of a few dropsof solution at the edge of the electrodes. Capillary forces weresufficient to draw the solution onto the entire electrode area. Solarillumination was simulated using the output of an Oriel 75 W xenon arclamp which was further attenuated using neutral density filters and a400 nm high-pass cutoff filter. The light intensity after filtering wasadjusted to 100 mW cm⁻² (ca. 1 sun) at the distance of the photoanodeusing a Molectron PowerMax 500A power meter. The current output of eachcell was recorded in the dark and under solar illumination whilesweeping the voltage between ca. 0.8 and −0.2 V using the sameinstrumentation as was used for cyclic voltammetry. The data thusobtained was digitized using an Acer flatbed scanner and tsEditdigitizing software on a computer running under Windows® 98.

Results and Discussion

Ligands

Cobalt complexes of tert-butyl substituted, in particularpara-substituted, bipyridine or terpyridine ligands gave a quite goodshort circuit photocurrent densities (J_(sc)) and open circuitphotovoltages (V_(oc)). It is believed that the difference in mediatorbehavior of t-butyl-substituted polypyridine ligands was related to thesteric bulk of the t-butyl group. Three types of polypyridine ligandswere examined: 2,2′-bipyridines, 1,10-phenanthrolines and2,2′:6′,2″-terpyridines. Alkyl substituents having a range of stericrequirements were examined. Since the electron-donating effect of allsimple alkyl substituents is essentially the same (e.g., methyl, ethyl,t-butyl, etc., see Wade, L. G. Organic Chemistry; 2nd ed.;Prentice-Hall, Inc.: Englewood Cliffs, N.J., 1991; Chapter 17), all ofthe complexes of a given ligand-type (i.e., bipyridine, phenanthrolineor terpyridine) were expected and found to have very similar E_(1/2)values for the relevant Co(II/III) couple. In addition, several othertypes of bulky substituents were examined which have significantlydifferent electronic effects. This group included aryl substituents andstrongly electron-withdrawing ester and amide groups. These latter twotypes of substituents make the ligands electron-deficient and producecobalt complexes with significantly more positive E_(1/2) values;consequently, the maximum theoretically possible V_(oc) is likewisegreater. Hagfeldt et al., Acc. Chem. Res., 2000, 33, 269.

Spectral Properties

All of the complexes under consideration exhibit similar UV-visabsorption spectra. Each of the Co(II) complexes has a weak absorptionband centered at ca. 440–450 nm. The onset of the ligand-based π-π*transition occurs in the UV above 350–380 nm for each of the ligands.Molar extinction coefficients (ε_(λmax)) for the band at 440–450 nm wereobtained from Beer's law plots of standard solutions of each Co(II)complex. Table 1 summarizes the data for a set of representativecomplexes. The most intense visible absorption is for ttb-terpy²⁺ withε₄₅₀=1.4×10³ M⁻¹ cm⁻¹. The remaining complexes all exhibit ε_(λ) ₄₄₀₋₄₅₀values that are approximately an order of magnitude smaller. In allcases, the visible absorbance of the Co(III) form is almostimperceptible and partial oxidation of solutions of any of the Co(II)complexes reduces the overall absorbance. For the sake of comparison,the ε_(λ) ₄₄₀₋₄₅₀ value for I₃ ⁻ is ca. 2×10³ M⁻¹ cm⁻¹; therefore,except for ttb-terpy²⁺ that has a comparable absorbance, considerablyless visible light is absorbed by all of the remaining cobalt complexesat similar concentrations.

TABLE 1 Spectral properties of representative cobalt(II) complexesComplex λ_(max) (nm) ελ_(max) (M⁻¹cm⁻¹) ttb-terpy 450 1.4 × 10³ dtb-bpy440 1.4 × 10² d3p-bpy 440 1.1 × 10² dn-bpy 440 1.1 × 10² bdb-amd 440 1.5× 10²Electrochemical Studies

Electrochemical characterization of these complexes revealed anelectrode surface dependence to the electron-transfer kinetics. Eachcomplex was examined by cyclic voltammetry on three different workingelectrodes: glassy carbon, gold, and platinum. FIG. 2 shows nine cyclicvoltammograms (CVs) representing three different complexes (rows) on thethree different working electrode surfaces (columns). The vertical axisin these CVs was converted to current density to normalize for thedifferent electrode areas. Table 2 contains the measured electrochemicalparameters for the complete set of complexes.

TABLE 2 Electrochemical properties of cobalt complexes Glassy CarbonGold Platinum ΔE_(P) ΔE_(P) ΔE_(P) Complex E_(1/2) (mV) (mV) E_(1/2)(mV) (mV) E_(1/2) (mV) (mV) te-terpy −103 200 −138 111 −140 156ttb-terpy −229 77 −234 75 (a) (a) bpy −7 86 −10 60 −7 60 4,4′-dmb −139171 −149 110 −79 290 5,5′-dmb −99 115 −103 60 −100 64 tm-dmb −217 123−225 57 −141 326 dtb-bpy −139 271 −177 86 (a) (a) dp-bpy −107 94 −109 69−81 162 d3p-bpy −58 116 −60 82 (a) (a) dn-bpy −147 80 −143 76 −125 221phen 80 198 73 87 81 153 phen-phen −87 60 −84 75 −71 163 dtb-est 174 103242 398 257 631 bdb-amd 217 101 222 86 291 442 (a) No discernablecathodic peak

The results found for 4,4′-dmb and dtb-bpy (FIG. 2, top and middle rows,respectively) are typical of complexes with ligands containing alkylsubstituents in the 4 and 4′ (or equivalent) positions. Of the threeelectrodes, gold electrodes exhibit the most reversible and ideallyshaped CVs. Glassy carbon electrodes also produce quasi-reversiblevoltammograms, although less reversible than gold. Quite unexpectedly,the voltammetry on platinum electrodes is quite irreversible with largeanodic and cathodic peak separations (ΔE_(p)) or, in some cases, peaksthat are so broad as to be indistinguishable as peaks. Of themetal-ligand complexes shown in FIG. 1, the voltammetry of the dtb-estcomplex (FIG. 1, bottom row), is most reversible on glassy carbon withΔE_(p) increasing on gold and platinum.

Complexes whose ligands are either un-substituted or are substitutedonly in the 5 and 5′ positions with methyl groups exhibited differentbehavior (see Table 2). In general, there is a less surface dependence.Gold and platinum electrodes give nearly reversible voltammograms whilethe CV's on glassy carbon are quasi-reversible.

In general, it is believed that the shapes of the quasi-reversible wavesindicate that, in cases where the heterogeneous electron transfer isslow, the transfer coefficient, α, is considerably greater than 0.5. SeeBard et al., Electrochemical Methods; 1^(st) ed.; John Wiley & Sons: NewYork, 1980; Chapter 3. In other words, it is believed that forequivalent overpotentials the heterogeneous reduction of the Co(II)complex is considerably faster than the corresponding oxidation of theCo(II) species. While the electrode-dependent electron-transfer kineticsare presently not fully understood, there is a rough empiricalcorrelation between the solution voltammetry of a complex and itsperformance as a redox mediator in a DSSC. The complexes that exhibitreversible or nearly reversible voltammetry on all three electrodes(i.e., gold, platinum, and glassy-carbon) are generally poor mediators;i.e., they give low J_(sc) values. The voltammetric results also suggestthat, while platinum is the cathode of choice for the I⁻/I₃ ⁻ redoxmediator, it may not be the optimal choice for cobalt complex-basedmediators. Likewise, while carbon is a poor cathode with the I⁻/I₃ ⁻redox mediator system, it may be acceptable for any of the cobaltsystems considered here.

Initial Screening of Mediators

To qualitatively and quantitatively compare these cobalt complexes asmediators, DSSCs were assembled using identical photoanodes andcathodes. Due to the varying solubility of the complexes, theconcentration of mediator in solution was kept low. This resulted indevices with less than optimal performance, but allowed for comparisonsin cell performance as a function of mediator structure.

As with the electrochemical observations, there were distinctdifferences in cell performance based on the identity of the ligandsubstituents and in what positions they were located. Mediators based onphen, phen-phen, bpy, 5,5′-dmb, or tm-bpy yielded almost nophotocurrent. Mediators composed of 4,4′-dmb and te-terpy resulted in avery modest photoeffect, but both V_(oc) and J_(sc) were very low. Bothdtb-est and bdb-amd gave V_(oc)>0.55 V but J_(sc) that were ca. <10%that of the best cobalt-based systems. The remaining complexes showedbetter promise as potential efficient electron-transfer mediators.

Solvents and Cathode Materials

In surveying a number of potential low volatility solvents, MPN and gBLwere found to work well with all of the cobalt mediators. For any givenconcentration of mediator, gBL was generally a superior solvent, in thatthe fill factor (FF) was improved over the same cell made with MPN asthe mediator solvent. However, in some cases the mediators were moresoluble in MPN, and in those cases the higher concentration of mediatormade for better cell performance.

Gold cathodes gave higher J_(sc) than platinum since, for efficientmediators, the reduction of Co(L)₃ ³⁺ (e.g. dtb-bpy³⁺, FIG. 2) is muchfaster on gold than platinum. For all the efficient cobalt complexmediators, cells assembled using gold cathodes generally gave betterperformance than those assembled with platinum. However, platinum gavebetter results than might have been anticipated from the voltammetry.There was no evidence that the cobalt complexes were corrosive towardsthe gold surface. In fact, the same gold cathode was used throughout thecourse of these experiments, and remained substantially unchanged.

Based on the CV results, it is expected that a carbon cathode shouldalso work well in these cells. Cathodes consisting of SnO₂:F glasscoated with a thin layer of graphite nanoparticles were prepared. Thesecarbon-coated cathodes worked well even outperforming platinum. In someinstances, however, carbon-coated cathodes were not as stable forextended periods. In general, a stable carbon cathode functions well incells based on electron-transfer mediators of the present invention.

Photoaction Spectra

Photoaction spectra-incident photon-to-current conversion efficiency(IPCE) versus wavelength—of N3 bound to nanocrystalline TiO₂ films inthe presence of different electron-transfer mediators in MPN solutionsare shown in FIG. 3. The performances of the photoelectrochemical cellare observed to be dependent on the composition of the electrolytesolution. A conversion efficiency of ca. 80%, in correspondence to themetal-to-ligand charge-transfer absorption maximum of N3 was obtained inthe presence of 0.25 M LiI/0.025 M I₂. With the cobalt complexmediators, an excellent performances were observed when solutions ofttb-terpy²⁺/ttb-terpy³⁺ (ca. 55% IPCE) and dtb-bpy²⁺/dtb-bpy³⁺ (ca 50%IPCE) were used. In other cases, the phen, te-terpy, and 44′-dmbcomplex-based mediators exhibited maximum IPCE values in the range of10–20%.

Open-Circuit Voltage

FIG. 4 shows V_(oc), E_(1/2) and J_(sc) data for five different cobaltbipyridine electron-transfer mediators plotted against the number ofcarbons in the substituents. In each case, the substituent is either anaryl or alkyl group that is appended at the 4 and 4′ positions. Thesemeasurements were all made using MPN as the solvent. The mediators wereall 125 mM in Co(II)L₃₁ and 13 mM in Co(III)L₃ and no other significantcations were present and the solution contained no pyridine type bases.As shown in FIG. 4, there is a steady increase in the value of V_(oc)with the number of carbons in the ligand's substituents, which appearsto be asymptotically approaching a limiting value. In the most generallyaccepted description, V_(oc) is the difference of the quasi-Fermi levelof electrons at the negative electrode and the “holes” at the positiveelectrode. See, for example, Hagfeldt et al., Acc. Chem. Res., 2000, 33,269; Cahen et al., J. Phys. Chem. B, 2000, 104, 2053; and Huang et al.,J. Phys. Chem. B, 1997, 101, 2576. This latter term is essentially theNerstian potential of the cobalt couple at the cathode. All of the4,4′-alkyl and 4,4′-aryl substituted bipyridine complexes herein havesimilar E_(1/2) (within ca. 60 mV), so it is expected that they will allyield approximately the same V_(oc), all else being equal. The fact thatV_(oc) varies by ca. 400 mV over this collection of mediators indicatesthat something must shift the Fermi energy of the electrons in the TiO₂.Two of the most obvious candidates are shifts in the conduction bandedge of the TiO₂ or differences in the rates of electron/Co(III)L₃recombination. With I⁻/I₃ ⁻, numerous studies have demonstrated thatV_(oc) depends on the size of the countercation of iodide. See, forexample, Kelly et al., Langmuir, 1999, 15, 7047; and Enright et al., J.Phys. Chem., 1994, 98, 6195. This effect is ascribed to a shift in theconduction band edge energy of TiO₂ upon adsorption and/or intercalationof cations; and the magnitude of this shift is related to thecharge-to-radius ratio of the cation. The complexes considered in FIG. 4are of different sizes and the trend in V_(oc) is in the correctdirection to be consistent with this model (i.e., V_(oc) increases withlarger radius). Furthermore, Nusbaumer et al. (J. Phys. Chem. B, 2001,105, 10461) have shown that a related cobalt complex-based mediator doesadsorb on the TiO₂ surface. While this is generally in qualitativeagreement with the model, it does not stand up to a more quantitativeanalysis. With I⁻/I₃ ⁻, the typical shift in V_(oc) upon changingbetween Li⁺ and Cs⁺ is less than 200 mV (at comparable concentrations)while their ionic radii differ by almost a factor of ×3. Liu et al.,Sol. Energy Mater. Sol. Cells, 1998, 55, 267. The difference in V_(oc)between bpy and dn-bpy mediated cells is ca. 400 mV but their radiidiffer by, at most, a factor ×2. Thus, it is believed that thevariations in V_(oc) arise from differences in the recombination ratebetween Co(III)L₃ and photoinjected electrons, which is expected todecrease as the bulk of the substituents increases.

Lithium Ion Effect

As considered above, the presence of small countercations (most notablyLi⁺) in mediator solutions of I⁻/I₃ ⁻ lowers V_(oc). Concomitantly,J_(sc) increases, and the net result is an overall improvement in cellefficiency (η). Kelly et al., Langmuir, 1999, 15, 7047. There is ampleexperimental verification that adsorbed Li⁺ lowers the energy ofacceptor states in the TiO₂; and this fact is the most commonly invokedexplanation for the cation-induced decrease in V_(oc). Id. The origin ofthe increased J_(sc) upon addition of Li⁺ to the I⁻/I₃ ⁻ is less clear.Photoinjection of electrons from the excited dye occurs with near unityquantum efficiency irrespective of the cation. Since the photocurrent isdetermined by the difference between the photoinjection rate and theoverall recombination rate, any increase in J_(sc) must lie incation-induced changes in recombination rates (irrespective of the typeof mediator system).

In principle, Li⁺ could decrease recombination in at least three ways:(1) by accelerating the rate of oxidized dye reduction by I⁻, (2) byslowing the rate of direct combination of electrons with the dye or (3)it could slow the rate of recombination of electrons with I₃ ⁻. Grätzelet al. (J. Phys. Chem. B, 2000, 104, 1791) showed that Li⁺ (and othercations with high charge-to-radius ratios) greatly accelerate the rateof reaction between I⁻ and adsorbed photooxidized N3. For the I⁻/I₃ ⁻system this is largely irrelevant to J_(sc) because, at usual I⁻concentrations, the rate of reaction between oxidized N3 and I⁻ is fasteven in the absence of Li⁺. Id. Consequently, neither processes (1) nor(2) above is believed to be a significantly factor in determiningJ_(sc). Thus, it is believed that the recombination reaction ofelectrons with I₃ ⁻ is the dominant factor. The Li⁺-induced lowering ofthe acceptor state energies in the TiO₂ could result in adriving-force-based decrease in the rate of I₃ ⁻ reduction at thephotoanode. Alternately, and most probably, adsorbed cations affect therate and or mechanism of the heterogeneous electron transfer in someundetermined way.

The addition of Li⁺ to solutions of the cobalt-based mediators alsoincreases J_(sc) significantly, and FIG. 5 shows the current-voltageresponse of cells demonstrating this effect. The analogous threeprocesses considered above remain relevant. As discussed in detail belowin the discussion of V_(oc), it is believed that Li⁺ has a marked effecton the recombination rate with Co(III)L₃.

In contrast to I⁻/I₃ ⁻, Li⁺ increases V_(oc) with all of the efficientcobalt mediators. Since it is unlikely that the presence of the cobaltcomplex in solution would alter the effect of Li⁺ in lowering the energyof the TiO₂ acceptor states (conduction band or surface states), it isbelieved that the Li⁺-induced increase in V_(oc) comes from some othersource. This effect must be large enough to offset the band shift tolower energy. Li⁺ has a negligible effect on the E_(1/2) of theCo(II/III)L₃ couple at the cathode; thus, it is believed that suchfactor is not the origin of the increase in V_(oc). Thus, as with theincrease in J_(sc), it is believed that factors directly tied to therecombination processes discussed above is responsible for the observedeffect.

It is believed that the recombination reaction at the SnO₂:F contact isaffected by Li⁺. On a bare SnO₂:F electrode of the same type used as theTiO₂ current collector, the overpotential for dtb-bpy³⁺ reduction isseveral hundred millivolts more negative in the presence of 0.25 M Li⁺and 0.10 M tetrabutylammonium ion (TBA⁺) than in 0.10 M TBA⁺ alone.

Pyridine Effect

The effect on V_(oc) of adding 4-t-butylpyridine to a cobalt mediatorsolution parallels the behavior with the I⁻/I₃ ⁻ mediator system—amodest improvement results. As is evident from FIG. 5, there is also asmall increase in J_(sc) that is not typical of the I⁻/I₃ ⁻ mediatorsystem. Huang et al., J. Phys. Chem. B, 1997, 101, 2576. When both4-t-butylpyridine and Li⁺ are present in solution, the increase inV_(oc) is significantly greater than for either alone (see FIG. 5). Theeffect of added 4-t-butylpyridine on the energy of the conduction bandand any other acceptor states in the TiO₂ is believed to be the same aswith I⁻/I₃ ⁻. Additionally, the same effect was observed whether the4-t-butylpyridine was added directly to the mediator solution or whetherthe photoanode was pre-treated by soaking in a solution of it.

Optimized Mediators

To fabricate DSSCs with higher η and FF, the composition andconcentration of the other mediators were surveyed. For dtb-bpy,d3p-bpy, and dp-bpy, it was found that good mediators were formed fromsaturated solutions of the corresponding Co(II) complexes and 0.5 Mlithium triflate. The solubility limit in gBL at ambient temperature isless than 0.5 M for dtb-bpy and less than 0.3 M for d3p-bpy and dp-bpyin MPN. The oxidized form of these complexes are considerably lesssoluble than the corresponding Co(II) complexes under these conditions,and oxidation of 10% of the saturated solutions led to precipitation.

The dn-bpy and ttb-terpy-based electron-transfer mediators show anexcellent performance at less than saturated concentrations. In somecases, above ca. 0.3 M, addition of more mediator actually lowers the ηof a cell. For dn-bpy, addition of the complex causes a significantincrease in the viscosity of the solution. The self-exchange rates ofcobalt complexes of this type are known to be slow (Szalda et al., J.Phys. Chem., 1983, 22, 2372; and Newton, M. D. J. Phys. Chem. 1991, 95,30), thus it is believed that the process of mediation is diffusioncontrolled. Under high viscosity conditions, this diffusional transportof mediator between dye-sites and the cathode is hindered and thus maybecome the limiting factor in determining J_(sc). This is consistentwith the behavior observed, i.e., as the concentration of dn-bpy isincreased, J_(sc) first increases, then decreases. For cells containinga ttb-terpy-based mediator, similar behavior is observed, however, it isbelieved that the peaking of J_(sc) values is due to the high ε_(λmax)observed for this complex—and since viscosity is not significantlydifferent even at high concentrations of mediator.

Table 3 lists the data obtained for DSSCs fabricated using cobaltmediators and comparable I⁻/I₃ ⁻ mediators. All the cobalt complexmediated cells suffer from a V_(oc) that is 100–200 mV less than thecomparable I⁻/I₃ ⁻ mediated cells. Nonetheless, the performance of thesecells is still quite good, and the η relative to a comparable I⁻/I₃ ⁻mediated cell (η_(rel)) is greater than 50% for d3p-bpy and dtb-bpymediated cells. Cells containing the dtb-bpy-based mediators haveexhibited an excellent performance, and FIG. 6 shows the current-voltageresponse of such a cell, which exhibits a FF of 62% and η_(rel) of 82%.

TABLE 3 Photoelectrochemical properties of DSSCs containing cobaltcomplex-based mediators. V_(oc) J_(sc) FF η^(b) η_(rel) ^(c)Mediator^(a) Solvent (Volts) (mA cm⁻²) (%) (%) (%) 0.25 M dn-bpy, 0.25 MLiTriflate gBL 0.43 0.89 59 0.22 31 0.25 M dp-bpy, 0.2 M LiClO₄ MPN 0.400.97 49 0.19 27 0.25 M ttb-terpy, 0.5 M LiTriflate MPN 0.40 1.71 48 0.3245  0.2 M d3p-bpy, 0.2 M LiClO₄ MPN 0.47 1.47 59 0.41 58 0.25 M dtb-bpy,0.5 M LiTriflate gBL 0.51 2.40 47 0.57 80 0.25 M LiI, 0.03 M I₂ MPN 0.602.03 58 0.71 — Sat'd dtb-bpy, 0.5 M LiTriflate^(d) gBL 0.44 4.82 62 1.3082  0.5 M LiI, 0.05 M I₂ ^(d) MPN 0.57 5.32 52 1.58 — ^(a)Theconcentrations of complex given are for the total cobalt; each solutionis 9:1 Co(II):Co(III). A gold cathode was used for all cells containingcobalt complex mediators; a platinum cathode was used in I⁻/I₃ ⁻mediated cells. These mediator solutions also contained 0.2 M4-t-butypyridine (see “d” below for exceptions). ^(b)All efficiencymeasurements were carried out under 100 mW cm⁻² (~1 sun) illumination.^(c)This is the efficiency relative to a comparable I⁻/I₃ ⁻ mediatedcell. ^(d)The photoanodes in these cells were constructed with aceticacid prepared TiO₂, and after dyeing, were soaked in 0.2 M4-t-butypyridine in ACN just prior to use.Conclusions

Metal-ligand complexes of the present invention are excellentelectron-transfer mediators for use in DSSCs. Cyclic voltammetricstudies have shown a significant surface dependence of theelectron-transfer kinetics. The electrochemical results showed that insome cases gold and carbon outperform platinum as cathode materials inthese cells. Furthermore, these mediators show no tendency to becorrosive, enabling the use of metallized SnO₂:F electrodes required inlarge-area DSSCs.

Photoelectrochemical measurements of cobalt complex mediated cellsrevealed that in some cases addition of lithium salts improves theirperformance significantly. It is believed that the Li⁺ effect observedwith electron-transfer mediators of the present invention is dueprimarily to a reduction in the recombination rate between Co(III)L₃ andthe electrons in TiO₂ and/or the SnO₂:F collector.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

1. A metal-ligand complex of the formula:L_(a)-M wherein a is 2; M is cobalt; and L is a ligand of the formula:

wherein each R¹ is independently alkyl, cycloalkyl, haloalkyl,heteroaryl, carboxy, or amide.
 2. The metal-ligand complex according toclaim 1, wherein each R¹ is independently alkyl.
 3. The metal-ligandcomplex according to claim 2, wherein each R¹ is independently selectedfrom the group consisting of ethyl and tert-butyl.
 4. The metal-ligandcomplex according to claim 3, wherein R¹ is ethyl.
 5. The metal-ligandcomplex according to claim 3, wherein R¹ is tert-butyl.
 6. Adye-sensitized solar cell comprising: a cathode; a photoanode comprisinga dye-sensitizer; and an electron-transfer mediator operativelyconnected to said dye-sensitizer, wherein said electron-transfermediator is a metal-ligand complex of the formula:L_(a)-M-X_(b) wherein a is an integer from 1 to 6; b is an integer from0 to 5, provided the sum of a and b equal the appropriate total numberof ligands present on the metal M; M is a transition metal; each X isindependently a co-ligand; and each L is independently a substitutedpolypyridine ligand, wherein at least one of the substituent of thepolypyridine ligand is other than a methyl group.
 7. The dye-sensitizedsolar cell according to claim 6, wherein M is cobalt.
 8. Thedye-sensitized solar cell according to claim 6, wherein b is
 0. 9. Thedye-sensitized solar cell according to claim 8, wherein L is selectedfrom the group consisting of a bidentate polypyridine, tridentatepolypyridine, and a mixture thereof.
 10. The dye-sensitized solar cellaccording to claim 9, wherein a is 2 or
 3. 11. The dye-sensitized solarcell according to claim 10, wherein a is
 2. 12. The dye-sensitized solarcell according to claim 11, wherein L is a tridentate polypyridineligand of the formula:

wherein each R¹ is independently hydrogen, alkyl, cycloalkyl, aryl,haloalkyl, hereroaryl, carboxy, or amide provided at least one R¹ isother than hydrogen or methyl.
 13. The dye-sensitized solar cellaccording to claim 12, wherein each R¹ is independently hydrogen oralkyl.
 14. The dye-sensitized solar cell according to claim 13, whereineach R¹ is independently selected from the group consisting of ethyl andtert-butyl.
 15. The dye-sensitized solar cell according to claim 10,wherein a is
 3. 16. The dye-sensitized solar cell according to claim 15,wherein L is a bidentate polypyridine ligand of the formula:

wherein each of R² and R³ is independently hydrogen, alkyl, aryl,carboxy, amide, cycloalkyl, haloalkyl, or heteroaryl provided at leastone of R² or R³ is other than hydrogen or methyl.
 17. The dye-sensitizedsolar cell according to claim 16, wherein each of R² independentlyselected from the group consisting of hydrogen, alkyl, aryl, carboxy,and amide.
 18. The dye-sensitized solar cell according to claim 17,wherein each of R² is independently selected from the group consistingof hydrogen, alkyl, N,N-dialkyl amide, alkyl carboxy, phenyl, andhaloalkyl.
 19. The dye-sensitized solar cell according to claim 18,wherein each of R² is independently selected from the group consistingof hydrogen, methyl, tert-butyl, 3-pentyl, nonyl, N,N-dibutyl amide,tert-butyl carboxy, phenyl, and haloalkyl.
 20. The dye-sensitized solarcell according to claim 15, wherein L is a bidentate polypyridine ligandof the formula:

wherein each or R⁴ and R⁵ is independently hydrogen, alkyl, aryl,carboxy, amide, cycloalkyl, haloalkyl, or heteroaryl provided at leastone R⁴ or R⁵ is other than hydrogen or methyl.
 21. The dye-sensitizedsolar cell according to claim 20, wherein at least one of R⁴ is aryl.22. The dye-sensitized solar cell according to claim 21, wherein atleast one of R⁴ is phenyl.
 23. The dye-sensitized solar cell accordingto claim 22, wherein R⁵ is hydrogen.
 24. The dye-sensitized solar cellof claim 6, wherein the electron-transfer mediator comprises atransition metal-polypyridine ligand complex selected from a moiety ofthe formula:

and a mixture thereof, wherein M is a transition metal; each R¹ isindependently alkyl; each of R², R³, R⁴, and R⁵ is independentlyhydrogen, alkyl, aryl, carboxy, amide, cycloalkyl, heteroaryl, orhaloalkyl provided at least one of (R² or R³) and (R⁴ or R⁵) are otherthan hydrogen or methyl.
 25. The dye-sensitized solar cell of claim 24,wherein M is cobalt.
 26. The dye-sensitized solar cell of claim 24further comprising a lithium salt.
 27. A battery comprising a solarcell, a photoanode comprising a dye-sensitizer, and an electron-transfermediator, wherein said electron-transfer mediator is a metal-ligandcomplex of the formula:L_(a)-M-X_(b) wherein a is an integer from 1 to 6; b is an integer from0 to 5, provided the sum of a and b equal the appropriate total numberof ligands present on the metal M; M is a transition metal; each X isindependently a co-ligand; and each L is independently a substitutedpolypyridine ligand, wherein at least one of the substituent of thepolypyridine ligand is other than a methyl group.
 28. Aphotoelectrochromic device comprising a dye and an electron-transfermediator, wherein said electron-transfer mediator is a metal-ligandcomplex of the formula:L_(a)-M-X_(b) wherein a is an integer from 1 to 6; b is an integer from0 to 5, provided the sum of a and b equal the appropriate total numberof ligands present on the metal M; M is a transition metal; each X isindependently a co-ligand; and each L is independently a substitutedpolypyridine ligand, wherein at least one of the substituent of thepolypyridine ligand is other than a methyl group.